Fiber optic interferometric sensors rely on phase shifts induced between two arms of an interferometer by an external field (measurand). These sensors can be used as accelerometers by incorporating them with other components to build the accelerometers. In all previous applications where the fiber optic accelerometers are used, the field does not act directly on the fiber but rather on a transducing material to which one arm of the fiber interferometer is attached and, with very few exceptions, the strain induced in the transducing material is linearly proportional to the strength of the external field. Optical fiber attached to the material to measure strain directly therefore responds linearly to the field. It is desirable, on the other hand that the response be nonlinear.
Nonlinear response is desirable because the important frequency components of many measurands of interest lie, unfortunately, in the low frequency range, typically less than ten (10) Hertz (Hz), or cycles per second, where interference from environmental noise sources such as thermal and mechanical fluctuations, is most severe. As a result, the shot-noise-limited sensitivity of an accelerometer incorporating a fiber interferometer can be achieved in practice only at frequencies above 10-100 Hz. However, if the phase shift (sensor output) depended quadratically on the total measurand then the problems associated with low-frequency noise could be overcome by mixing the low-frequency measurand with a high-frequency dither supplied by the sensor itself. The low-frequency measurand then modulates the phase shift at the dither frequency and the measurement can be made at a frequency (the dither frequency) where environmental noise is acceptably low.